Our recent studies have demonstrated the presence of multiple, genetically independent forms of attention deficit/hyperactivity disorder (ADHD) which would be best studied individually. We have also identified a highly heritable continuum of Liability to attention problems. The long-range goal of this new application is the identification of genetic factors that predispose to the development of severe inattention problems. The specific goals of this application are to complete a whole genome linkage study and candidate gene analyses using quantitative trait locus (QTL) as well as multivariate variance components analyses of combined continuous trait and categorical phenotype definitions. The target sample is families in Australia that have already been identified and characterized for inattention problems (SWAN scale and DSM-IV ADHD symptoms). This sample of twin families appears representative of the general population of Australia and, hence, represents a community-based approach to studying this form of ADHD, which frequently does not come to clinical attention. We will use novel phenotyping approaches including a continuum assessment of inattention (the SWAN scale) and latent-class defined ADHD subtypes. Because data on the two key phenotypes have already been collected, we will be in a position to efficiently select the 1,200 families with greatest power to detect linkage or association out of the existing sample of 7,550 families. We anticipate having complete data on 1,000 of these families (both parents and an average of 4 siblings per family). A whole genome 10cM scan will be completed on this sample as will single nucleotide polymorphism (SNP) analysis of 15 well-documented candidate genes (including the DRD4, DRD5, DAT, SNAP-25 and CHRNA4 genes). This combined approach will determine the relevance of candidate genes to inattentive forms of ADHD in a population-based sample of families as well as have sufficient power to discover new chromosomal locations linked to inattentive ADHD. Furthermore, using an existing epidemiological sample of families will 1) provide a powerful test of the relevance of specific genes to the general population, 2) have sufficient power to discover new loci related to inattentive ADHD, 3) will avoid many of the potential confounds of other studies using clinic or advertisement-based sampling of ADHD cases, and 4) will test the general relevance of findings from the United States by the use of an independent and culturally diverse sample. This will be at a marked cost savings due to the employment of existing well-characterized samples and the long history of collaborative studies between investigators from Australia and the United States.